Base-catalyzed nitrito to nitro linkage isomerization of cobalt(III), rhodium(III), and iridium(III) pentaammine complexes

Article abstract of DOI:10.1021/ic50206a022

The nitrito ions [M(NH₃)₅(ONO)]²⁺ (M = Co(III), Rh(III), or Ir(III)) rearrange to their nitro forms [M(NH₃)₅(NO₂)]²⁺ in aqueous base according to the rate law k_obsd = k_s + k_OH[OH^-] where at μ = 1.0 M, 25.0°C: k_s = 6.9 (±0.2) × 10^-5 s^-1, ΔH_s^≠ = 95 ± 4 kJ mol^-1, ΔS_s^≠ = -4 ± 12 J K^-1 mol^-1, k_OH = 5.9 (±0.2) × 10^-3 dm³ mol^-1 s^-1, ΔH_OH^≠ = 110 ± 1 kJ mol^-1, ΔS_OH^≠ = +82 ± 4 J K^-1 mol^-1, M = Co; k_s = 9.6 (±0.2) × 10^-4 s^-1, ΔH_s^≠ = 85 ± 2.5 kJ mol^-1, ΔS_x^≠ = -18 ± 9 J K^-1 mol^-1, k_OH = 7.6 (±0.2) × 10^-3 dm³ mol^-1 s^-1, ΔH_OH^≠ = 96 ± 2 kJ mol^-1, ΔS_OH^≠ = +38 ± 6 J K^-1 mol^-1, M = Rh; k_s = 3.0 (±0.1) × 10^-5 s^-1, ΔH_s^≠ = 94 ± 2 kJ mol^-1, = 16 ± 5 J K^-1 mol^-1, k_OH = 2.7 (±0.1) × 10^-5 dm³ mol^-1 s^-1, ΔH_OH^≠ = 109 ± 2 kJ mol^-1, ΔS_OH^≠ = 34 ± 6 J K^-1 mol^-1, M = Ir. For the cobalt complex the pressure dependence of k_OH (ΔV^≠ = +27 (±1.4) cm³ mol^-1, Δβ^≠ = 5 (±2) cm^-3 kbar^-1 mol^-1) points to a conjugate base preequilibrium for the base-catalyzed path. The metal ion dependence of k_OH/k_s (95:9:1 for Co:Rh:lr) is also consistent with a conjugate base process. [Co(NH₃)₅(ONO)]²⁺ rearranges to [Co(NH₃)₅(NO₂)]²⁺ without exchange of oxygen with H₂¹⁸O, ¹⁸OH^-, or free N¹⁸O₂^- (1 M); both the k_s and k_OH paths are intramolecular. Up to 1 M OH there is <1% competitive hydrolysis to [M(NH₃)₅(OH)]²⁺. Both rate constants k_s and k_OH for M = Co decrease with increasing ionic strength (25°C, 10₅k_s = 6.9, 7.4, 10³k_OH = 5.9, 14.5; μ = 1.0, 0.1, respectively). cis- and trans-[Co(en)₂(ONO)₂]⁺ and [Co(en)₂(ONO)(NO₂)]⁺ rearrange by both pathways to the dinitro complexes without change in configuration, and (+)cis-[Co(en)₂(ONO)(NO₂)]⁺ rearranges without loss of chirality. For the k_OH reaction these results also differ from the normal substitution chemistry where there is some steric change. In addition to the O to N rearrangement, a much faster O to O scrambling process has been identified for the cobalt complex using ¹⁸O tracers.